arm: Add support for RCpc load-acquire instructions (ARMv8.3)

arm: Add support for RCpc load-acquire instructions (ARMv8.3)

Please note that at the moment these instructions behave like the
existing load-acquire instructions, which follow the more conservative
RCsc consistency model. This means that the new instructions are
_functionally_ correct, but the potential performance improvements
enabled by the RCpc model will not be experienced in timing
simulations.

Change-Id: I04c786ad2941072bf28feba7d2ec6e142c8b74cb
Reviewed-by: Andreas Hansson <andreas.hansson@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/11989
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>